Emergency communications in coastal waters

Jack Pare


If you have a medical or equipment emergency while boating in coastal waters, should you use a cellphone or a digital selective calling equipped marine VHF radio? Let’s look at the differences between these two communication methods.

Cellphones make handy backup devices for marine use. Most new phones have water-resistant cases and built-in GPS chips. With a compass app, you can sight down an edge of the phone, tap the compass rose on the screen and lock in a compass bearing—an instant hand-bearing compass.

If you’re inland and have adequate cell coverage, you can dial 911 and connect to an Emergency Communications Center equipped with Enhanced 911 software, which provides the operator with your latitude and longitude.

Using a cellphone navigation app, you can see nearby marine chart details on the phone’s small screen. However, you must zoom out to see the broader context of where your boat is. Zooming out also simplifies the screen display and may hide some hazards.

Despite their usefulness, cellphones aren’t designed for primary emergency communications on coastal waters. As one-to-one communication devices, cellphones can only communicate your situation to the party you dialed. If someone nearby could help, they won’t hear you, and the 911 Call Center won’t know they’re available. At best, the call center would notify the Coast Guard and a boat tow service of your situation. If you’re on an inland lake or pond, they might notify the local fire department.

Cellphones vs. VHF radio

Unlike cellphones, VHF radio provides one-to-many communications. Any nearby boaters should hear your distress call on Channel 16 or receive your automatic digital selective calling alert on Channel 70.

Theoretically, both cellphones and VHF radios are line-of-sight devices and, as such, could have a maximum range of 25 miles with high enough antennas. In its lowest band, a cellphone can transmit up to a 3-watt signal; however, by legal agreement to protect human health, higher capacity 4G and 5G signals are limited to 0.2 watts. This limits their effective range to 2 to 3 miles, at best.

By comparison, a handheld VHF radio can transmit 1 watt (for in-harbor communications, such as Channel 13 bridge-to-bridge) up to 5 watts for general communications and emergencies. Generally dependable up to about 5 miles, VHF radios have an ideal maximum range of 12 miles. Fixed-mount VHF radios can deliver 25 watts over a higher, more efficient antenna, providing a useful range of 20 to 25 miles. Big ships with high-mounted VHF antennas can communicate up to 60 miles over the open ocean.

When considering power and range, a cellphone can’t compare with a VHF radio. You could be 20 miles off the coast and expect to dependably reach the Coast Guard on the mainland with a VHF radio. With a cellphone, you’re limited to about 2 miles—if you can connect with a tower.

Cellphones have limitations

Designed for land-based customers, cell service uses a series of towers to provide continuity and avoid dropped calls. If a moving cellphone’s signal weakens, the device detects the next most powerful signal tower. After some behind-the-scenes data dialogue and an electronic hand-off, the cellphone talks to the next tower. If anything blocks either tower’s signal, the cellphone drops your call.

Concentrated in urban areas and spread along major roadways, cell towers are optimally placed to avoid terrain that can block the line-of-sight signal over land. Any cellular signal passing over coastal waters is incidental spill-over. Over coastal waters, the chance that a cell signal will be blocked by hilly terrain increases significantly since the system wasn’t designed for water-based customers. For example, in New England, terrain blocking is noticeable off the hilly areas around Cape Ann, Massachusetts, and the Maine coast between Portland and Canada.

Rescue 21 for the win

Coast Guard Sector Northern New England

By contrast, the U.S. Coast Guard has a Rescue 21 system specifically designed to nearly eliminate terrain blocking and provide a continuous watch for DSC distress alerts on Channel 70, with the capability to respond to distress alerts. That response would typically be on VHF Channel 16.

The accompanying figure shows antenna locations and VHF coverage for Coast Guard Sector Northern New England monitored by the Coast Guard base in South Portland, Maine. Note that the signal to and from the northernmost antenna at Quoddy Head is blocked eastward of Grand Manan Island, New Brunswick, Canada, but the Coast Guard isn’t concerned with coverage of Canadian waters. The other colored “rays” of terrain blockage are filled in by the colors of at least one other VHF antenna. Since all Rescue 21 system VHF antennas are listening for your signal, there’s no gap in reception.

It’s OK to carry a cellphone for backup communications, as a hand-bearing compass and a charting device, but boaters should rely on VHF radios for primary emergency communications.

For more information, check out a complete coverage list of all the USCG Rescue 21

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